This invention relates to a gas discharge light emission (or generation) apparatus type utilizing light emitted (or generated) discharge as a source of light and a method of driving the same.
A plasma display panel (PDP) is known as gas discharge light emission apparatus.
There are AC and DC type PDPs, and various PDPs are disclosed in, for instance, a literature I entitled "Electronics Technology Series", No. 4, Latest Display Device/Apparatus Technology, 1985. As an example of a PDP, a DC type PDP disclosed in the Literature I will be described.
FIG. 1 is a perspective view, partly broken away, schematically showing the construction of the prior art DC type PDP. The illustrated prior art PDP has a rear substrate 1, on which are provided a plurality of ribbon-like anodes 2 and also DC and AC auxiliary anodes 3 and 4. These anodes extends parallel to one another, and partitioning members 5 are provided between adjacent anodes 2 to 4. The PDP further comprises a plurality of ribbon-like cathodes 6 and an auxiliary DC cathode 7. These regular and auxiliary cathodes 6 and 7 perpendicularly confront and are spaced apart from the regular and auxiliary anodes 2 to 4.
First discharge cells for display are formed in areas where the cathodes 6 intersect and confront the anodes 2. Second discharge cells for firing discharge display for the first discharge cells are formed in areas where the cathodes 6 intersect and confront the AC auxiliary discharge anode 4. Further, third discharge cells for firing discharge for the second discharge cells are formed in areas where the DC auxiliary discharge cathode 7 and DC auxiliary discharge anode 3 intersect and confront one another.
The cathodes 6 are formed at positions corresponding to the first discharge cells with small holes 8 as priming holes. Further, for reducing statistical delay for discharge in the discharge cells, ions and excited atoms generated as a result of firing discharge are supplied from the second discharge cells to the first discharge cells.
Further, for stabilizing light emission by discharge in the discharge cells, the anodes 2 are connected through resistors RD to data signal lines, the The DC auxiliary anode 3 is connected through a resistor RA1 to a power supply EA, the AC auxiliary anode 4 is connected through a resistor RA2 to the power supply EA, and the DC auxiliary cathode 7 is connected through a resistor RC to ground.
The PDP further comprises a front substrate 9. The front and rear substrates 9 and 1 are sealed together by a sealing section (not shown), thus sealing gas capable of discharge, for instance a gas mixture of Ne and Ar (referred to as discharge gas medium), between the substrates.
FIG. 2 shows a two-dimensional matrix wiring structure of this prior art DC type PDP. Referring to FIG. 2, a plurality of (i.e., m) anodes (generally designated at 2) are individually designated at 201, 202, . . . 2m, a plurality of (i.e., n) cathodes (generally designated at 6) are individually designated at 601, 602, . . . 6n, data signals supplied to the anodes 2 are designated at A1, A2, . . . , Am, and scanning signals supplied to the cathodes 6 are designated at C1, C2, . . . , Cn.
The data signals A1, A2, Am are supplied through resistors RD1, RD2, . . . , RDm to the anodes 2.
The circles shown in FIG. 2 designate discharge cells.
FIG. 3 shows scanning signals supplied to the cathodes, and FIG. 4 shows discharge light emission timings in the discharge cells FIG. 4 shows an example of waveforms of scanning signal and data signal. As shown in FIG. 3, the scanning signals Cl to Cn each consist of an "on" state (for instance 0-V voltage state) for emission of light in the discharge cells and an "off" state (for instance 80-V voltage state), provided by cathode pre-bias voltage V.sub.Cpb, for extinguishing light in the discharge cells. The duration T of the "on" state constitutes a unit time. The "on" states of the individual scanning signals Cl to Cn are applied sequentially to the individual cathodes 601 to 6n (FIG. 2).
As shown in FIG. 4, the data signals (only data signal A1 being shown), like the scanning signals, each consist of an "on" state (for instance 180-V voltage state) and an "off" state (for instance 100-V voltage state) provided by anode pre-bias voltage V.sub.Cpb. The duration T of the "on" state again constitutes a unit time. The data signals A1 to Am are applied to the respective anodes 201 to 2n (FIG. 2).
When displaying or printing characters or drawings with light emission in the discharge cells, the scanning signals Cl to Cn are sequentially applied to the cathodes 601 to 6n, thus sequentially turning on these cathodes (see FIGS. 2 and 3). The data signals A1 to Am are applied to the anodes 201 to 2n with their timings matched to the "on" state of the cathodes for causing selective light emission in the discharge cells. Thus, in a discharge cell, in which light is to be emitted, both the anode and cathode are turned on, and a voltage in excess of a discharge-starting voltage (for instance 130 V) is applied between the two electrodes. In a discharge cell, in which no light is to be emitted, on the other hand, both the anode and cathode are held "off", and a voltage lower than the discharge-starting voltage is applied between the two electrodes.
Referring now to FIGS. 4 and 2 as an example, during a period S1-S2, an "on" state scanning signal is applied to the cathode 601, an "off" state scanning signal is applied to the cathodes 602 to 6n, and an "on" state data signal is applied to the anode 201. Thus, in the discharge cell in the area of intersection between the cathode 601 and anode 201 light is emitted a by discharge, while no discharge light emission takes place in the discharge cells in the areas of intersection between the cathodes 602 to 6n and anode 201.
During periods S2 S3 and S5 S6, an "off" state data signal is applied to the anode 201. During this time, therefore, no discharge light emission is caused in the discharge cells in the areas of intersections between the anode 201 and cathodes 601 to 6n regardless of application of an "on" state scanning signal to the cathodes 601 to 6n.
During a period S3-S4, an "off" state scanning signal is applied to the cathode 601, while an "on" state data signal is applied to the anode 201. During this period, discharge light emission does not take place in the cell at the area of intersection between the cathode 601 and anode 201, but instead it takes place in the cells in the areas of intersections between the cathodes among cathodes 602 to 6n, an on which "on" state scanning signal prevails, and anode 201
It will be understood from the foregoing that and "off" state data signal may be applied to the anode 201 if light emission in the cells in the area of intersection between the anode 201 and cathodes 601 and 6n is not desired. In this case, if an "off" state data signal is applied to the adjacent anode 202, crosstalk (i.e., erroneous discharge) is caused between the anode 202 and cathodes among cathodes 601 to 6n facing the anodes 201 and 203, on which an "off" state scanning signal prevails. To prevent such crosstalk, the height of the partitioning members 5 is set to prevent erroneous discharge between adjacent anodes
In the meantime, in the light emission by gas discharge, it is possible to select a wavelength range suited to wavelength sensitivity characteristics of various photosensitive medium by appropriately selecting the composition of the sealed gas. For this reason, it is thought to utilize PDP or like gas discharge type light emission devices as optical heads of electrographic (i.e., optical) printers The prior art structure of such a prior art gas discharge type discharge head will now be described briefly.
FIG. 5 shows a main wiring of a prior art gas discharge type optical head (also referred to as optical print head).
In this illustrated of gas discharge type, optical head a plurality of, i.e., eight, anodes 1001 to 1008 are provided on one of the pair substrates in a row extending in a main scanning direction, and also a plurality of, i.e., 64, cathodes 1101, 1102, . . . , 1164 are provided in parallel to one another on the other substrate. The two substrates are sealed together in a spaced-apart relation to each other such that the anodes and cathodes perpendicularly cross one another, and discharge gas is sealed between the substrates.
Each anode is common to a suitable number of (for instance eight) cathodes, and this electrode set (for instance the anode 1001 and cathodes 1101 to 1108) constitutes an electrode block A plurality of electrode blocks are arranged side by side in the scanning direction to constitute an electrode block line.
On the other substrate (not shown) noted above are further provided a plurality of, i.e., eight, scanning electrodes 1201 to 1208. The cathodes and scanning electrode are connected to one another such that like cathodes in the individual blocks are connected only to a corresponding scanning line, for instance the cathodes 1101 to 1164 are commonly connected to the scanning electrode 1201. Matrix connection by multi-layer wiring thus is provided. With this construction, discharge light emission can be caused independently for each block.
Data signals A1 to A8 are applied through resistors R1 to R8 to the anodes 1001 to 1008, and scanning signals C1 to C8 are applied to the scanning electrodes 1201 to 1208. Light emission is caused in the discharge cells, in which both the input signals are in the "on" state. For preventing crosstalk, partitioning member 12 are provided between adjacent electrode blocks (i.e., adjacent anodes).
In a display, the picture element density may be 3 dots/mm. In the optical head, however, the picture element density is desired to be as high as 8 to 16 dots/mm. To meet such a high picture element density requirement, the inter-electrode distance between adjacent electrodes should be small. Partitioning members are formed by thick film printing techniques. However, it is very difficult to form partitioning members in narrow inter-electrode spaces as noted above such that they have a height which is sufficient to prevent erroneous discharge and such that they are accurately positioned. At any rate, inferior yield and high cost are inevitable. Without any partitioning member, however, crosstalk occurs.
Now, a prior art plasma light emission electrode structure constructed as an optical head will be described.
FIG. 6 is a schematic fragmentary sectional view showing a prior art plasma optical head.
In this prior art optical head, a single anode 15 is formed on a substrate 14, while a plurality of parallel cathodes 17 are formed on another substrate 16. These substrates 14 and 16 are sealed together with lead glass or a like sealing agent such that their electrode formation sides face each other. A narrow space 18 is formed between the substrates 14 and 16 sealed together, and rare gas mainly composed of Ne (sealed gas) is sealed in the space. Insulating films 19 are formed on &he electrode formation sides for restricting light emission areas More specifically, they have apertures or windows 20 and 21 exposing corresponding portions of the anode 15 and cathodes
FIG. 7 schematically shows mainly the positional relation between the anode and cathodes of this prior art optical head As is shown, the parallel cathodes 17 face the stripe-like anode 15 at a predetermined distance therefrom, thus forming respective discharge cells.
The anode 15 is connected through a resistor 22 to the positive potential side of a high voltage power supply 23 at +V (of 180 to 220 V), and the cathodes 17 are connected through a driver circuit 24 to the negative potential side (0 V) of the power supply 23. By selectively applying a voltage between the anode 15 and cathodes 17, discharge light emission is caused in the discharge cells constituted by the selected ones of electrodes 15 and 17.
FIGS. 8(a) to 8(c) are views for explaining light emission areas (also referred to as light emission section). In these figures, light emission areas 25 are shown with scattered dots. In the case of FIG. 8(a), the electrodes 15 and 17 are made of a non-transparent metal electrode. Since the electrodes 15 and 17 are overlapped in the direction of taking out light, discharge light has to be taken out from the edge of the anode 15. In other words, at the time of light emission, light in a portion of the light emission area 25 that is not concealed by the anode 15 is taken out.
In the case of FIG. 8(b), the anode 15 is a transparent electrode. In this case, the light emission area 25 is not concealed by the anode 15. Therefore, light can be taken out from the entire light emission area 25.
In the case of FIG. 8(c), adjacent light emission areas 25 partly overlap each other
With the above plasma light emission apparatus constructed as an optical head, however, if the anode 15 and cathodes 17 are non-transparent electrodes (FIG. 8(a)), light is taken out from only a portion of each light emission area 25. Actually, only about 50% of the total amount of light emitted in the light emission area 25 is taken out from the area.
Where the anode 15 is a transparent electrode, the amount of light taken out from the light emission area 25 can be increased. However, the light emission area of discharge light emission formed in the neighborhood of the cathode is wider in plan view than the cathode, and this means an increase of the print dot size. In addition, when the picture element density is increased, the light emission area 25 of adjacent discharge cells are liable to partly overlap each other in plan view. That is, adjacent light dots are not definitely separated, so that a clear print can not be obtained. To avoid partial overlap of light from adjacent light emission area 25, the cathodes 17 have to be spaced apart sufficiently Doing so, however, imposes a limitation on the resolution of the optical head.
Further, in a plasma light emission apparatus constructed as a two-dimensional display, like the optical head (optical printing head), a reduction of the amount of taken-out light is liable to occur, or an overlap of light from adjacent light emission areas imposes a limitation on the resolution. Further, in the case of a two-dimensional display where partitioning members are provided around individual discharge cells to prevent the spread of light from each light emission area, although overlap of light can be prevented by the partitioning members, the partitioning members themselves limit the picture element density.
Now, a prior art method of driving a plasma optical head having two rows of discharge cells will be described. In the optical head, two cathode rows each consisting of a plurality of cathodes are provided for each anode such that one row of cathodes is deviated by one-half pitch relative to the other row of cathodes in the direction of rows.
FIG. 9 schematically shows the electrode structure and circuit wiring of a prior art plasma line head (optical head) Referring to FIG. 9, the width direction of a photosensitive medium is a main scanning direction A, and a direction of movement of the photosensitive medium is a auxiliary scanning direction B. Symbols 1 to 8 designate respective eight cathodes groups (of four cathodes each), into which the cathodes are divided by multi-layer wiring. The cathodes (generally designated at 26) in the same group are electrically connected together.
In this example, four anodes (generally designated at 27) are provided such that each is common to eight cathodes 26, i.e., to one cathode each of the eight cathodes groups 1 to 8. Four electrode blocks are thus provided. A discharge cell as a print cell is formed at each of the areas where the anodes 27 and cathodes 26 confront one another. In this example, the print cells are arranged in a staggered fashion in the auxiliary scanning direction B.
Symbols (1) to (4) are ordinal numbers of the anodes 27, and hence electrode blocks, given sequentially in the main scanning direction A.
FIG. 10 is a timing chart for explaining the method of driving the plasma line head having the above construction.
This method is a cathode eight-division driving method. The cathodes 26 are divided by multi-layer wiring into eight groups, and a voltage is applied sequentially to the cathode groups in the order of 1, 3, 5, 7, 2, 4, 6, 8, 1, . . . for a pulse period t1 as unit time, during which the scanning signal is in the "on" state, for each cathode group.
Meanwhile, a voltage is applied for pulse period t1 to anodes among the anodes 27 corresponding to print cells, in which light emission is to be caused, in synchronism with the timing of scanning of the cathodes 26.
Slight light emission pulse t1 is also applied at all time to the anodes 27 in synchronism with the timing of scanning of the cathodes 26. In this way, an effect of priming for light emission is obtained without any auxiliary electrode.
The phase "effect of priming" denotes an effect provided by a light emission element, in which there is or was discharge light emission, to reduce a discharge start voltage and delay time in the discharge start in its own or in the next light emission element to cause light emission.
FIG. 11 is a circuit diagram of the driver circuit.
Anode data 29 is supplied to a shift register 28. The anode data covers several anodes, and after its input the output of the shift register 28 is supplied to a latch 30, and substantially simultaneously the output is supplied to an anode driver 31 for conversion to a high or low voltage, which is supplied as print information to print cells (discharge cells) 32.
Cathode data 33 on the cathode side is supplied to a shift resister 34, and a waveform for eight cathode groups is sequentially passed through a cathode driver 35 for conversion to a high or low voltage supplied to the print cells 32.
Slight light emission pulse t2 is supplied from a direct set terminal 36 to the anode driver 31. If the signals from the direct set terminal 36 and latch 30 are ORed, the voltage of slight light emission pulse t2 may be applied when no printing is done, and the voltage of light emission pulse t1 when printing is done.
As soon as anode data 29 is completely shifted to the latch 30, next anode data 29 turns to be supplied to the shift register 34. For supplying data to the shift register 34 or latch 30, anode clock 38 and latch clock 38 are used. Cathode clock 39 has a like function.
With the above construction and method of driving discharge light emission is caused in print cells, in which the light emission pulse t1 prevails on anode 27 and cathode 26 simultaneously, and the photosensitive medium is generated by the emitted light.
At this time, discharge light emission is caused in current print cells 32 for slight light emission pulse t2 to provide a priming effect to the next print cells to cause discharge light emission In this way, the priming effect is shifted progressively.
The deterioration of contact of print due to discharge light emission for slight light emission pulse t2 can be sufficiently accommodated in a permissible contract range by minimizing the slight light emission pulse.
While the prior art plasma discharge light emission apparatus is described in connection with a plasma line head, the prior art plasma display panel utilizes discharge cells of a driving method like that for the plasma line head as light emission elements, and hence display cells.
Like the case of the plasma line head, the cathodes are scanned by sequentially applying a light emission pulse signal to them, while a light emission pulse signal based on light emission/non-emission data is selectively applied to the anodes in synchronism with the timing of the cathode scanning, thus effecting matrix driving of the display cells.
Further, to provide a priming effect a slight light emission pulse signal is applied to the anodes in synchronism with the cathodes scanning timing.
In the above prior art driving method, however, the priming effect is received irregularly because of zig-zag sequence of cathode driving.
More specifically, there are cases of unnecessary shift and failure of necessary shift of the priming effect. To describe the former case in detail, it is assumed that a light emission mode is set in a print cell (1)-8 constituted by the anode number (1) and cathode group 8 in FIG. 9 so that light emission pulse is applied to the anode 27 of anode number (1) and cathodes 26 in the cathode group 8, and in the next cycle the light &lt;mission mode is set in print cell (1) 1 but not in print cell (2) - 1, that is, a light emission pulse is applied to the anode 27 of anode number (1) and to cathodes 26 in the cathodes group 1 but not applied to the anode 27 of the anode number (2). In this case, light emission is caused for the light emission pulse period in the print cell (2) - 1 which is not in the light emission mode. This takes place because the priming effect of the cathode in the print cell (1) - 8 causes discharge light emission between the anode 27 of the anode number (1) and cathode in the print cell (2) - 1.
As the latter case, the priming effect is considered, which is provided by light emission for the slight light emission pulse period in, for instance, print cell (1) - 7. This priming effect is to be received by the print cell (2) - 2. However, since the print cells (1) - 7 and (2) - 2 are distant from each other, a failure of a sufficient shift of priming effect is liable to occur, resulting in a failure of the light emission. This also applies to the relation between the print cells (2) - 7 and (3) - 2.
As has been shown, in the prior art, erroneous discharge and failure of discharge are prone, deteriorating the print quality in the case of the plasma line head and deteriorating the display quality in the case of the PDP.
Further, the prior art plasma line head has the following problems.
Dots printed by print elements are spaced apart very finely in the direction of movement of photosensitive medium (e.g., in the auxiliary scanning direction B). Therefore, it is impossible to move the photosensitive medium in the form of a drum intermittently and accurately using a stepping motor after the end of printing of one line (i.e., printing of dots corresponding in number to 8 times the number of anodes) for the printing of the next line.
For this reason, in the above prior art driving method, driving of the print elements is effected as parallel driving for each anode from one end to the other end thereof, while rotating the photosensitive drum at a constant speed in the direction of auxiliary scanning. In this method, however, time corresponding to the number of provided light emission pulse cathode group is passed from the first printing till the last printing of one line. Therefore, there is produced between the result of print by the print cell (1) - 1 and that of the print cell (1) - 8, for instance, a step corresponding to the product of the time difference between the timing of light emission in the print cell (1) - 1 and that of subsequent light emission in the cell (1) - 8 and speed of movement of the photosensitive medium. As a result, a sawtooth-like print pattern is produced.
Because of the formation of such a sawtooth-like print pattern, in the case of characters where blank portions are defined by horizontal and vertical lines crossing one another such as Japanese characters, deterioration of the visual quality of printed characters is liable to occur.